Duchenne Muscular Dystrophy (DMD) is an X-linked inherited muscle disease caused by mutations of the dystrophin gene. While increased membrane fragility has been considered as a primary pathogenic mechanism of DMD, accumulated evidence suggests that the loss of sarcolemmal nNOS also contributes to the dystrophic process. For example, the mild variant of DMD, Becker Muscular Dystrophy (BMD), results from in-frame mutations of dystrophin, thus expressing the truncated dystrophins in muscle. Many of the truncated dystrophins in BMD lose the ability to tether neuronal nitric oxide synthase (nNOS) to the sarcolemma. Clear evidence shows that deficiency of sarcolemmal nNOS causes muscle ischemia and predominantly contributes to the characteristic symptoms of BMD, such as muscle cramp and pain on exercise, muscle fatigue and reduced exercise endurance. However, current therapies are less effective for muscle ischemia and the resultant symptoms.
Since loss of sarcolemmmal nNOS is responsible for BMD symptoms, recovering sarcolemmal nNOS has been suggested as a plausible approach to treat BMD and DMD. It has been known that sarcolemmal localization of nNOS is sustained by dystrophin, and the inventors' previous studies, through systemic structure-function analysis, have found that dystrophin spectrin-like repeats 16 and 17 (R16/17) are required for sarcolemmal distribution of nNOS. Basically, dystrophins that contain R16/17 show membrane expression of nNOS while those without R16/17 do not. However, the current mini-genes or micro-genes with repeats R16/17 still require the retention of other dystrophin domains. For example, even the smallest nNOS binding dystrophin (ΔR2-R15/ΔR18-R23/ΔC) engineered in previous studies still carries the NT and CR domains, H1, H4, R1 and R24. Furthermore, though the mini-genes or micro-genes previously identified are reduced in size as compared to existing gene therapy, more significant size reduction is desired to increase efficacy of delivery of the therapy.
Therefore, there is a need to provide a series of new biological materials containing certain domains/sections of the dystrophin repeats R16 and R17 for anchoring nNOS to the sarcolemma as a new therapy/treatment for DMD and BMD.